Note: Descriptions are shown in the official language in which they were submitted.
CA 02379643 2002-03-28
TITLE OF THE INVENTION:
PUMPING SYSTEM AND METHOD FOR PUMPING FLUIDS
BACKCaROUND OF THE INVENTION
The present invention gE:nerally relates to systems and methods for
transferring
fluids from a vessel to another location or' an end user, and more
particularly to a system
and method for pumping cryogenic fluids from a vessel to another location or
an end user.
In general, past attempts to optimize cryogenic pump systems have fallen short
of
providing an economical and effective means of cooling the pump and minimizing
product
waste. Most cryogenic pumps in service have no insulation on the inlet line or
on the vapor
return line. These systems have proven to be wasteful of cryogen, often
venting and losing
substantial product. To ensure that these systems operate without cavitating,
the systems
generally have a vacuum jackei:ed sump at the inlet of the pump that acts as a
phase
separator. Also, the pump must be cooled down to an appropriate level with a
minimum of
wasted product.
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One way to reduce product losses; is to insulate the inlet and/or vapor return
lines.
This not only helps to reduce losses, but also improves pump performance.
However, there
are drawbacks to insulating the piping. If l:he vapor return line is not
insulated, there will be
liquid cryogen in this line which will boil off and add to the vent losses of
the system. For
vacuum jacketed piping, the cost of the piping can exceed the cost of the pump
itself. If
insulated with foam insulation, trre foam is subject to thermal cycling which
damages the
foam and draws in moisture. Freezing of water inside the insulation can result
in higher
heat leak rates than an uninsulai:ed line.
Others have attempted to overcome these deficiencies in the prior art. Various
prior
art systems which have attempted to reduce product losses and/or overcome the
other
above-described deficiencies are discussed below.
One prior art method is to submercfe the pump in a supply tank or vessel so
that the
pump is always cold. Losses for this type of system are primarily due to heat
leak of the
vessel and heat generation of the pump.
U.S. Pat. Nos. 4,472,946 (Zwick) and 4,860,545 (Zwick, et al.) disclose a
cryogenic
storage tank with a built-in submerged pump that is kept in a continuously
cooled down
state by the cryogen stored in the tank such that pumping may be commenced
immediately.
This approach attempts to reduce the loss of cryogen through boil-off by
minimizing the
heat leak path from the environment into the cryogen caused by the presence of
the pump
inside the tank. This is done by providing an insulated cryogenic storage
vessel with a
pump mounting tube extending into the vessel and immersed in the cryogen. The
outer
surface of the pump mounting tube within the vessel is insulated so as to
minimize the heat
leakage from the pump mounting tube to the cryogen surrounding the tube.
However, there
are several drawbacks to this design, which in general is impractical. First,
there is the
requirement of a special tank iii which to install the pump. Second, to repair
the pump, the
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tank pressure must be vented and the pump removed and warmed up before repairs
can
be made. Overall, the costs associated with this design are unacceptable.
U.S. Pat. No. 5,819,544 (Andonian) discloses a high pressure pumping system
for
pumping cryogenic liquid frono a low pressure holding cylinder to a high
pressure gas
cylinder (or other high pressure utilization system). 'the system includes a
high pressure
piston pump having a unidirectional flow input and a unidirectional flow
output immersed in
the cryogenic liquid in a low pressure pump container that is fed cryogenic
liquid from the
low pressure holding cylinder. The pressure in the pump container is
maintained so that
driving the pump piston pumps cryogenic liquid from the bulk tank to the high
pressure
utilization system. Although this design is more economical than the cryogenic
storage tank
with built-in pump by Zwick, it has other problems. For example, the smaller
tank must be
filled periodically. This results in vent losses due to blowing down of the
vessel and line
heating. Further complications are added because of the controls needed to
accomplish
tank filling without the pump having to shut down.
U.S. Pat. No. 5,218,827 (I'evzner) discloses a method and apparatus for
supplying
liquified gas from a vessel to a pump with subcooling so as to avoid
cavitation during
pumping. No attempt is made to rninimize ,product losses, only to provide a
subcooled liquid
to the pump. Problems associated with vent losses are largely ignored.
U.S. Pat. No. 5,537,828 (l3orcuch, et al.) discloses a temperature-based
cryogenic
pump cooldown system wherein the suction or input conduit to the cryogenic
pump and the
cryogenic pump itself are sequentially cooled prior to pumping. This system
also ignores
problems associated with vent losses, focusing primarily on how the pump is
effectively
cooled down and how that cool clown is monitored and controlled.
U.S. Pat. No. 5,411,3'74 (Gram) discloses a cryogenic fluid pump system and
method of pumping cryogenic fluid. The system is intended primarily for LNG,
although it
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discusses other cryogenic fluids. It does not discuss insulating the lines,
nor does it discuss
a conventional vapor return line. The pump is required to pump vapor and
liquid separately
out of the inlet line. Cooldown of the pump is accomplished by recirculating
the cryogenic
fluid back to the top of the supply tank, which is not an uncommon practice.
U.S. Pat. No. 5,353,849 (Sutton, et al.) discloses another method of operating
a
cryogenic pump, which is complicated by additional methodology used to meter
the
cryogenic fluid. The method used to cool down the pump is similar to that in
U.S. Pat. No.
5,411,374 (Gram). A liquid sensor (e.g., a temperature probe) indicates when
cryogenic
liquid has gone through the pump. When the probe indicates liquid downstream
of the
pump, there is a time delay before the pump is started.
U.S. Pat. No. 5,160,769 (Garrett) discloses a method to minimize vent losses
in
cryogenic pump systems. This patent teaches a type of purged cryogenic pipe
insulation
particularly for cryogenic fluids that are less than 77 Kelvin (-321
°F).
U.S. Pat. No. 3,630,639 (Durron, et al.) also discloses a method to minimize
vent
losses in cryogenic pump systE:ms. Specifically, this patent teaches the use
of an
automatically controlled vent valve in a vent line connected to the suction
line in a cryogenic
pumping system. The vent valve is in an open position during the cooldown
cycle and is
moved to a closed position after the system has reached desired operating
conditions.
' Blowby gas which leaks around the piston of the pumping system provides the
pressure for
closing the vent valve. The vent valve contains an orifice through which the
blowby gas
bleeds and returns to the storagE; vessel for the cryogenic fluid being
pumped.
It is desired to have an apparatus and a method that will minimize product
losses
associated with the operation of cryogenic pumps by minimizing heat leak
during the
pumping cycle and by more efficient means of cooling down the pump to
cryogenic
temperature.
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It is further desired to have an apparatus and method which use an insulation
for
cryogenic pipe that is more durable and effective than conventional foam
insulations by
making use of gas vaporized during normal operation of a cryogenic tank which
would
otherwise be wasted.
It is still further desired to have an apparatus and a method to ensure that
the
cryogenic pump has a minimum net positive suction head (NPSH) at the suction
without the
need for elevating the cryogenic supply tank.
It also is desired to have an improved apparatus and method for transferring a
fluid
from a vessel to an end user which overcomes the difficulties and
disadvantages of the prior
art to provide better and more advantageous results,
BRIEF ~~UMMARY OF THE INVENTION
The invention is an apparatus and an method for transferring a fluid from a
vessel.
The invention also includes a method for controlling cooldown of a pump.
A first embodiment of the apparatus includes a pump having an inlet and an
outlet,
a first conduit having a first end and a second end, and a first control means
in fluid
communication with the pump and a having an open position and a closed
position. The
first end of the first conduit is in fluid comrnunication with the vessel and
the second end is
in fluid communication with the inlet of the pump. The first control means
alternates
between the open position and the closed position, whereby a stream of the
fluid flows into
the inlet of the pump from the first conduit when the first control means
first alternates to
the open position, the first control means alternates to the closed position
and at feast part
of the stream of the fluid vaporizEa in the pump thereby forming a vaporized
portion of the
fluid, and a stream of the vaporized portion of the fluid flows out of the
pump outlet when
the first control means alternates again to the open position.
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There are several variations of the first embodiment of the apparatus. In one
variation, the fluid is a cryogenic, fNuid. In another variation, the
vaporized portion of the fluid
is transferred to the vessel.
A second embodiment of the apparatus is similar to the first embodiment but
includes a temperature sensor. 'The sensor senses a temperature of at least a
portion of
the fluid in the pump or at least a portion of the fluid upstream or
downstream of the pump.
A third embodiment of the apparatus is similar to the first embodiment but
includes
a phase separator in fluid communication with the first conduit at a first
location between
the first end and the second end. The phase separator is adapted to transfer a
vapor
stream from the first conduit to the vessel.
A fourth embodiment of the apparatus is similar to the third embodiment but
includes
a first layer of insulation, a second layer of insulation, a source of purge
gas, a second
conduit, and a second control means. The first layer of insulation
peripherally surrounds
the first conduit. The second layer of insulation is spaced apart from and
peripherally
surrounds the first layer of insulation, thereby forming a first space between
the first and
second layers of insulation. The second conduit has a first end in fluid
communication with
the source of purge gas and a second end in fluid communication with the first
space. The
second control means controls a flow of the purge gas from the source to the
first space.
A fifth embodiment of the apparatus is similar to the first embodiment but
includes
a first layer of insulation, a second layer of insulation, a source of purge
gas, a second
conduit, and a second control means. The first layer of insulation
peripherally surrounds
the first conduit. The second layer of insulation is spaced apart from and
peripherally
surrounds the first layer of insulation, thereby forming a first space between
the first and
second layers of insulation. The second conduit has a first end in fluid
communication with
the source of purge gas and a r>econd end in fluid communication with the
first space. The
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second control means controls a flow of the purge gas from the source to the
first space.
There are several variations of the fifth embodiment. In one variation, the
source
of the purge gas is in the vessel. In anothE~r variation, the first layer of
insulation is a closed
cell cryogenic foam. In yet another variation, at least part of the purge gas
is selected from
the group consisting of hydrragen, helium, argon, oxygen, hydrogen, carbon
dioxide,
hydrocarbons, and mixtures thereof, they hydrocarbons being selected from the
group
consisting of methane, ethane, butane, propane and mixtures thereof.
A sixth embodiment of the apparatus includes a pump having an inlet and an
outlet,
a first conduit having a first end and a second end, a phase separator, a
first layer of
insulation, a second layer of insulation, a source of purge gas, a second
conduit, and a
control means. The first end of 'the first conduit is in fluid communication
with the vessel
and the second end is in fluid communication with the inlet of the pump. The
phase
separator is in fluid communication with the first conduit at a first location
between the first
end and the second end. The phase separator is adapted to transfer a vapor
stream from
the first conduit to the vessel. The first layer of insulation peripherally
surrounds the first
conduit. The second layer of insulation is spaced apart from and peripherally
surrounds the
first layer of insulation, thereby forming a first space between the first and
second layers of
insulation. The second conduit has a fir~~t end in fluid communication with
the source of
purge gas and a second end in fluid communication with the first space. The
control means
controls a flow of the purge gas from the source to the first space.
As with the apparatus, there are various embodiments of the method for
transferring
a fluid from a vessel. The first embodiment of the method comprises multiple
steps. The
first step is to provide a pump having an inlet and an outlet. The second step
is to provide
a first conduit having a first end and a second end, the first end being in
fluid
communication with the vessel and the second end being in fluid communication
with the
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inlet of the pump.. The third step is to provide a first control means in
fluid communication
with the pump and having an open position and a closed position. The first
control means
is adapted to alternate between the open position and the closed position,
whereby a
stream of the fluid flows into the inlet of the pump from the first conduit
when the first control
means first alternates to the open position, the control means alternates to
the closed
position and at least a part of the stream of the fluid vaporizes in the pump
thereby forming
a vaporized portion of the fluid, and a stream of the vaporized portion of the
fluid flows out
of the pump outlet when the first control means alternates again to the open
position. The
fourth step is to alternate the first control means between the open position
and the closed
position. The fifth step is to transmit a first stream of the fluid from the
first conduit to the
inlet of the pump when the first control mE~ans is first in the open position.
The sixth step
is to transmit a first stream of the vaporized portion of the fluid out of the
pump outlet when
the first control means is again in the open position.
In one variation of the first embodiment of the method, the fluid is a
cryogenic fluid.
A second embodiment of the rnethod includes an additional step of transmitting
at least a
portion of the stream of vapor to the vessel.
A third embodiment of the method is similar to the first embodiment, but
includes the
additional step of sensing a temperature of at least a portion of the fluid in
the pump or at
least a portion of the fluid upstream or downstream of the pump.
A fourth embodiment of the method is similar to the first embodiment, but
includes
two additional steps. The first additional step is to provide a phase
separator in fluid
communication with the first conduit at a first location between the first end
and the second
end, the phase separator being adapted to transfer a vapor stream from the
first conduit to
the vessel. The second additional step is to separate a stream of a vapor from
at least a
portion of the stream of the fluid.
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A fifth embodiment of the method is similar to the first embodiment, but
includes six
additional steps. The first additional step is to provide a first layer of
insulation peripherally
surrounding the first conduit. The second additional step is to provide a
second layer of
insulation spaced apart from an peripherally surrounding the first layer of
insulation, thereby
forming a first space between the first and second layers of insulation. The
third additional
step is to provide a source of a purge gas. The fourth additional step is to
provide a second
conduit having a first end in fluid communication with the source of purge gas
and a second
end in fluid communication with the first space. The fifth step is to provide
a second control
means for controlling a flow of the purge gas from the source to the first
space. The sixth
step is to transmit a controlled flow of the purge gas from the source of the
purge gas to the
first space.
There are several variations of the fifth embodiment of the method. In a first
variation, the first layer of insulation is a closed cell cryogenic foam. In a
second variation,
the source of the purge gas is in the vessel. In another variation, the purge
gas is selected
from the group consisting of nitrogen, helium, argon, oxygen, hydrogen, carbon
dioxide,
hydrocarbons, and mixtures thereof, the hydrocarbons being selected .from the
group
consisting of methane, ethane, butane, propane and mixtures thereof.
A sixth embodiment of the method includes multiple steps. The first step is to
provide a pump having an inlet and an outlet. The second step is to provide a
first conduit
having a first end and a second end, the first end being in fluid
communication with the
vessel and the second end being in fluid communication with the inlet of the
pump. The
third step is to provide a phase separator in fluid communication with the
first conduit at
a first location between the first end and the second end, the phase separator
adapted to
transfer a vapor stream from the first conduit to the vessel. The fourth step
is to provide a
first layer of insulation peripherally surrounding the first conduit. The
fifth step is to provide
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a second layer of insulation spaced apart from and peripherally surrounding
the first layer
of insulation, thereby forming a first space between the first and second
layers of insulation.
The sixth step is to provide a source of a purge gas. The seventh step is to
provide a
second conduit having a first end in fluid communication with the source of
purge gas and
a second end in fluid communication with the first space. The eighth step is
to provide a
control means for controlling a flow of the purge gas from the source to the
first space. The
ninth step is to transmit a first stream of the fluid from the vessel to the
first conduit. The
tenth step is to separate a stream of a vapor from at least a portion of the
first stream of the
fluid. The eleventh step is to transmit a controlled flow of the purge gas
from the source of
the purge gas to the first space.
Another aspect of the invention is a method for controlling cooldown of a pump
having an outlet and an inlet in communication with a source of a fluid. The
method
includes multiple steps. The first step is to provide a control means in fluid
communication
with the pump and having an c7pen position and a closed position. The control
means is
adapted to alternate between the open position and the closed position,
whereby a stream
of the fluid flows into the inlet of the pump from the source when the control
means first
alternates to the open position, the first control means alternates to the
closed position and
at least part of the stream of the fluid vaporizes in the pump thereby forming
a vaporized
portion of the fluid, and a stream of the vaporized portion of the fluid flows
out of the pump
outlet when the control means alternates again to the open position. The
second step is
to alternate the control means between the open position and the closed
position. The third
step is to transmit a stream of the fluid from the source of the inlet of the
pump when the
control means first is in the open position. The fourth step is to transmit a
stream of the
vaporized portion of the fluid out of the pump outlet when the control means
is again in the
open position.
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There are several variations of the method far controlling cooldown of a pump.
In
one variation, the fluid is a cryogenic fluid. In another variation, the step
of alternating the
control means between the open and closed positions includes five sub-steps.
The first
sub-step is to designate a setpoint for a variable temperature, the
temperature to be
determined in the pump or at a. location upstream or downstream of the pump.
The second
sub-step is to provide a sensing means for sensing the temperature. The third
sub-step is
to move the control means to the open position, thereby allowing a stream of
the fluid to
flow into the inlet of the pump. The fourth sub-step is to move the control
means to the
closed position when a desigroated amount of fluid has flowed into the inlet
of the pump.
The fifth sub-step is to move i:he control means back to the open position
when the
temperature sensed by the sensing means is less than the set point.
Another embodiment of the method for controlling cooldown of a pump is similar
to
the first embodiment of that method but includes the additional step of
sensing a
temperature of at least a portion of the fluid in the pump or at least a
portion of the fluid
upstream or downstream of the pump.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention will be described by way of example with reference to the
accompanying drawings, in which:
Figure 1 is a schematic representation illustrating one embodiment of the
present
invention;
Figure 2 is a schematic representation illustrating a second embodiment of the
present invention;
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Figure 3 is a schematic representation illustrating a third embodiment of the
present
invention; and
Figure 4 is a schematic representation illustrating the multiple layers of
insulation
used in the present invention.
DETAILED DESCRIPTION OF THE INVENTION
The invention is a pumping system and a method for operating the pumping
system
to minimize the amount of product lost by the system during operation and
cooldown. The
invention includes various features which, when combined, minimize the loss of
product.
Although the invention may be used with various types of fluids, it is
particularly useful with
cryogenic fluids.
Cryogenic temperatures are measured on the absolute or Kelvin scale in which
absolute zero is 0 K. The cryogenic temperature range is from about -
150°C (-238°F) to
absolute zero (-273°C or -460°F), or about 123 K to 0 K.
The invention is described herein with regard to cryogenic fluids; but persons
skilled
in the art will recognize that the invention is not limited to use with
cryogenic fluids. (For
example, the invention could be used with relatively cold fluids having
temperatures higher
than the temperatures of "cryogenic fluids," but which would change phase in
the system
in a manner similar to that described below for cryogenic fluids.) A double-
acting, two-stage
pump that works particularly well with the system and method of this invention
is discussed
in a patent application being filed concurrently with this application and
which is entitled
"Double-Acting, Two-Stage Pump" (Air Products and Chemicals, Inc.'s docket
number
06112USA),
The key features of the invention, when used with cryogenic fluids, are:
1 ) An inlet line supplying liquid cryogen to a pump is insulated and is
purged
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using gas from a supply t<~nk that has boiled off and would otherwise be
wasted by venting to atmosphere. Alternatively, a separate source of inert
gas may be used.
2) The inlet line has a phase separator that only allows vapor to return to
the
supply tank such that the vapor return line need not be insulated.
3) The pump is cooied down by automatically opening, then closing, in an
alternating manner, a valve (pump unloader valve) downstream of the pump
so that liquid can be brought into the pump and allowed to boil off slowly,
thus making morE: efficient use of the refrigeration value of the cryogen.
This is monitored by a temperature probe or sensor mounted on the pump
assembly. Alternatively, the temperature may could be mounted in the
upstream or downstream piping. The pump unloader valve normally
discharges to atmosphere, although the pump also can be made to run
during this cycle and the pump unloader valve can return product to the
supply tank.
One embodiment of the system 10 is illustrated in Figure 1. Alternate
embodiments
are shown in Figures 2 and 3.
Referring to the system 10 in Figure 1, the cryogenic fluid 12 is stored in a
supply
tank 14 which is encased in a larger tank 16. The fluid is transferred from
the supply tank
to a pump 20 by an inlet line 1:i. A suction valve 22 in the inlet line may be
used to control
the flow of fluid from the supply tank to the pump via the inlet line. A phase
separator 24
in the inlet line separates vapor from the liquid in the fluid. The liquid
flows to the pump
inlet, and the vapor is returned to the supply tank via a vapor return line
32. The pump 20
is cooled down by automatically opening a.nd then closing in an alternating
manner a pump
unloader valve 22 located downstream of the pump outlet. The pump unloader
valve is in
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the open position and liquid flow; into the pump when the temperature reaches
a setpoint,
as measured by the temperature probe 38. The pump unloader valve moves to the
open
position and the vapor which boiled off the liquid in the pump is vented to
the atmosphere
28. The liquid discharged from the pump is transmitted to another location 30
in the system
which may be an end user, a tank, etc. (not shown).
As shown in Figure 1, the inlet line 18 is insulated, and as shown further in
Figure
4, the insulation 34 actually comprises multiple layers. The first layer of
insulation 44 is a
closed cell cryogenic foam insulation capable of handling the low temperatures
of cryogenic
fluids. The second layer of insulation 46 preferably is an open cell foam
insulation, although
a closed cell type of insulation also is acceptable. Because this second layer
of insulation
typically does not have to handle lower temperature fluids as does the first
layer of
insulation, an open cell polyurethane foam insulation is preferred for the
second layer of
insulation. In the space between 'the first and second layers of insulation,
an inert gas, such
as nitrogen, argon or helium, is used for a purge. Many other gases could be
used for the
purge gas, including but not limited to carbon dioxide, oxygen, hydrogen, and
certain
hydrocarbons (e.g., methane, ethane, butane, propane and mixtures thereof).
Although the
inert and non-flammable gases are preferred, use of the other gases would be
feasible if
non-flammable types of insulation are used.
The purge gas permeates the second layer of insulation 46 (the open cell
foam), but
remains relatively stagnant around the first layer of insulation 44 ('the
closed cell foam). The
outer layer (third layer) of insulation 48 acia as a rain barrier and also is
used to contain the
purge gas. The purge gas is admitted to i:he space between the first and
second layers of
insulation via the conduit 42 connected to the supply tank 14 from which the
purge gas is
withdrawn. Flow of the purge gas is controlled by the insulation purge flow
control valve 36.
Figures 2 and 3 show alternate embodiments of the system 10. The alternate
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embodiment shown in Figure 2 is similar to the embodiment in Figure 1, except
that the
vapor from the pump unloader valve 26 is re-circulated to the top of the
supply tank 14. The
second alternate embodiment of the system 10 shown in Figure 3 is similar to
the
embodiment in Figure 1, except that the pump suction valve 22 is located
between the
supply tank 14 and the phase separator 24.
A key feature of the system 10 is the multi-layer design of the insulation 34.
The
insulation is most applicable to situations where a source of dry nitrogen or
other inert gas
is available that can be used for a purge where this gas otherwise might be
vented to
atmosphere and thus wasted. (cryogenic tanks supplying cryogenic pumping
systems
typically vent gas due to the heal: input to the tank which boils off liquid.
That gas can not
be consumed by the pump, and is often too great a quantity to simply fill the
volume of the
removed liquid and so it must be vented.
Another key feature of the system 10 is the use of a mechanical phase
separator
24 on the inlet line 18 near the pump 20, as shown in Figures 1-4. In the
preferred
embodiment, this device is a valve connected to a float which allows vapor
only (not liquid)
that boils off in the inlet line to travel back to the vapor space of the
supply tank 14. By
providing this device in the inlet line, the piping of the vapor return line
32 is greatly
simplified. First, there is no need for insulation on the vapor return line.
This reduces cost,
more than making up for the add~ad cost of the phase separator. Second, the
vapor return
line does not have to be carefully laid out to ensure that there are no liquid
traps in the line.
A liquid trap in the vapor return line can easily prevent vapor from rising up
the vapor return
line to the top of the tank, thus creating a bubble that forces liquid out of
the inlet line. The
result is that the pump could have gas at the inlet instead of liquid,
resulting in the pump not
being able to operate.
A third key feature of the system 10 is the method of cantrolling cooldown of
the
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pump 20. The system is controlled and monitored to minimize the amount of
product used
for cooldown of the pump. To get liquid into the pump, the pump unloader valve
26 opens
to atmosphere 28 downstream of the pump allowing liquid to flow into and
through the
pump. The pump unloader valve is then shut to allow this standing liquid to
boil off inside
the pump, thus cooling down the pump. The pump unloader valve is made to
operate in an
alternating manner as required to ensure that there is liquid inside the pump
for cooling.
When the pump temperature has reached a desired setpoint, the pump unloader
valve
opens again to vent any vapor inside the pump, and then the valve closes and
the pump
is allowed to run, Alternatively, 'vapor transmitted from the pump unloader
valve can be
routed back to the supply tank 14 at the top, the bottom, or another location
of the tank. At
the same time that the pump unloader valve is opened, the pump can be turned
on and the
fluid routed back to the supply tank. This alternative is shown in Figure 2
for the case
where the vapor transmitted fram the pump unloader valve is routed back to the
top of the
tank.
The pump unloader valve 26 is pulsed, rather than kept open. By doing this,
the
cryogenic liquid has more time to exchange heat with the pump 20 and the
piping, thus
using more of the refrigeration capacity of the cryogenic liquid.
Although illustrated and described herein with reference to certain specific
embodiments, the present invention is nevertheless not intended to be limited
to the details
shown. Rather, various modifications may be made in the details within the
scope and
range of equivalents of the claims and without departing from the spirit of
the invention.
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